In a multisite trunked radio communication system a radio can roam from site to site. The radio tries to operate at a site with the best received radio frequency (RF) level, or at a site which is specified as a preferred site for this radio (programmed to the radio on individual identity (ID) or talkgroup basis).
Normally more than one site can be received at different frequencies and the radio may select any of these sites according to a predetermined selection algorithm. The radio has to register at a site before it is allowed to operate there. Once registered the radio user may request services, such as dispatch calls, phone interconnect calls, etc., from the system.
This registration is done with a multisite controller that controls the whole trunked system. Such a multisite controller is available under the name "Smartzone Controller" from Motorola, Inc. The multisite controller maintains a database for each radio in the network. A radio (or talkgroup) may be permanently denied to operate at a site (individual or talkgroup disabled at that site). Some services may be selectively disabled for a radio for a site (e.g. interconnect calls, talkgroup calls, announcement calls, etc.).
Whenever a radio locks to a new site, it tries to register at that site by sending its individual ID and current talkgroup. The multisite controller checks if both an individual ID and a present talkgroup are disabled at that site for denying access and, if not, it grants the registration. If the registration request is denied the radio searches for another site. If the registration request is granted the radio remains operating at that site. When the user wishes to initiate any service request the radio sends its individual ID and the required service. The multisite controller checks if the requested service is enabled to that individual ID at that site. If the kind of service is allowed and there are resources available for that request (e.g. voice channels, interconnect channels, data channels, etc.) the service is granted.
In the event that the kind of service is allowed and there are no resources immediately available the user enters a busy queue, to be served when the resources become available. If the service is disabled a "service reject" code is sent. A new identical request from the radio will be handled by the multisite controller in the same way again. There may be cases in which radios will concentrate operating at some of the sites thereby overloading these site resources while other sites will be loaded only lightly. This may happen when the selected sites have strong transmission power (or their antennae are located high), or simply due to random selection which occasionally can yield uneven distribution. Once a radio operates at a site, the RF level of the adjacent sites needs to be significantly stronger to force an attempt to switch site (in order to prevent frequent site switching as the RF level is temporarily faded). This means that the duration of uneven site load condition may last a relatively long time. In the field of handover methods for cellular radiotelephones it is known to consider not only the received field strengths but also the actual traffic loading in each cell, for example in considering whether to hand over from a micro-cell to a macro-cell. These methods teach only about traffic distribution and not about registration distribution.
Previous multisite controllers have been somewhat rigid in operation, either totally accepting or rejecting registration requests and either totally accepting or rejecting requests for service (with service requests being put in a busy queue and granted later if not immediately granted). There is a need for an improved method of operation of a trunked, cellular or other radio system.